Low-protein diet therapy for Chronic Kidney Disease (which will be hereinafter referred to CKD) has been practiced for a long time. The mechanism has not still been clarified, but is thought to (1) reduce the total amount of the nitrogen compounds resulting from the protein, and decrease the glomerular loading, (2) suppress the production of uremic toxins which cause renal injury, (3) suppress the accumulation of phosphorous or potassium, (4) suppress the production of acids, and the like, by inhibiting the intake of the protein derived from diet. In recent years, the effect of the low-protein diet therapy on inhibiting CKD progression has also been proved in some clinical tests ((a) “The New England Journal of Medicine”, 1989, Vol. 321, No. 26, p. 1773-1777; (b) “The Lancet”, 1991, Vol. 337, No. 8753, p. 1299-1304; (c) “American Journal of Kidney Diseases”, 2003, Vol. 41, No. 3, p. S31-S34), and is established in the guidelines (Japan Society of Nephrology, “Evidence-Based CKD Guidelines 2009”, 2009, p. 40-49). Recently, it is suggested that the low-protein diet therapy prevent not only the pregression of the CKD but also diabetic nephropathy, chronic nephritis, nephrosis, gout kidney, hepatic encephalopathy, and the like, and thus, is now being practiced clinically. However, it is known that since the therapy, as seen from one point of view, requires specialized knowledge, close cooperation among a doctor, dietitian doctor and a patient is necessary, and further, since the low-protein diet itself is a special diet, it is difficult to practice the diet.
On the other hand, it is known that a compound which inhibits trypsin, which is one of enzymes classified as a serine protease, is useful for diseases involving this enzyme, such as, pancreatitis and gastroesophageal reflux disease, and camostat mesylate (which will be hereinafter described camostat) (A) (Patent Document 1) has been actually used for chronic pancreatitis and gastroesophageal reflux disease in clinical practice. Further, it has also been reported that camostat reduce the urinary albumin excretion in animal models with diabetes mellitus (“Nephron”, 1996, Vol. 74, No. 4, p. 709-712).

In addition, it has also been reported that trypsin is involved in the proliferation of influenza viruses, because it is necessary that hemagglutinin (HA) on the virus surface should be cleaved into two subunits of HA1 and HA2 by the trypsin in the airway or mucosal intestinal epithelium in order to obtain the infectivity of the virus, but by inhibition of the trypsin, the cleavage of this HA is suppressed and the virus loses infectivity, whereby the proliferation is suppressed. Therefor, a compound inhibits the trypsin can also be used as an anti-influenza drug (“Antiviral Research”, 2011, Vol. 92, No. 1, p. 27-36; (b) “Protease Groups of Individuals which Determine Susceptibility to Infection of Influenza Virus and Pathogenesis of Influenza-Associated Encephalopathy”, “The Japanese Journal of Pharmacology”, 2003, Vol. 122, p. 45-53).
As a compound exhibiting a trypsin inhibitory action, other than camostat, Compound (B) (Patent Document 2), Compound (C) (Patent Document 3), Compound (D) (Patent Document 4), Compound (E) (Patent Document 5), Compound (F) (Patent Document 6), Compound (G) (Patent Document 7), and Compound (H) (Patent Document 8) have been reported. However, there is no disclosure of the compound of the formula (I) or a salt thereof of the present application as described later in these documents.

(In the formula (C), A is —N(R1, R2) or the like, and R1 and R2 are each H, lower alkyl having 1 to 8 carbon atoms, aralkyl which may have a substituent, or the like. In the formula (D), X is an oxygen atom or a sulfur atom, and R is —C(O)N(R1)—(CH2)m-(1-azabicyclo[3.3.0]octan-5-yl) or the like. In the formula (E), R1 is a hydrogen atom or a halogen atom, and R2 is —OCOR3 or the like. In the formula (F), A is (CH2)n or a styrene group, R2 is —NH(CH2)nCOOR4, —NHCH—(—R5)—COOR4, —NH—C6H4—(CH2)p—COOR4, or the like, n is 0 to 5, p is an integer of 0 or 1, R4 is a hydrogen atom, lower alkyl, or a substituted or unsubstituted benzyl group, R5 is a substituted or unsubstituted benzyl group, a methoxycarbonylmethyl group, and the substituent of the substituted benzyl group means a halogen atom, a nitro group, a lower alkyl group, a hydroxy group, an alkoxy group having 2 to 6 carbon atoms, or the like. In the formula (G), X represents lower alkylene or the like, Y represents a carbonyl group or the like, A represents —NR6R7, R6 and R7 may be the same as or different from each other and each represents a hydrogen atom, a lower alkyl group which may have a substituent, or the like, or R6 and R7 may be bonded to each other to form a cyclic amino group which may have a substituent. In the formula (H), R2 is a substituted phenyl group, or the like, and R3 is any of various substituents. For the other symbols, refer to the corresponding patent publications.)
Furthermore, as a guanidino compound having an effect of inhibiting the production and release of inflammatory cytokines, Compound (J) (Patent Document 9) has been reported. However, there is no disclosure or suggestion of a specific compound as the compound of the formula (I) or a salt thereof of the present application as described later in these documents.

(wherein R is a guanidino group, an amidino group, or the like, A1, A2 and A3 are each a bond or the like, L is an arylene group or the like, X is —COO— or the like, M is an arylene group, a divalent heterocyclic group, which has at least one hetero atom selected from a nitrogen atom, a sulfur atom, or an oxygen atom, and may form a fused ring, or the like, R5 is a hydrogen atom or the like, R6 is —CR12R13—(CH2)m—R11 or the like, R12 and R13 are a hydrogen atom or the like, R11 is —COOR16, is R16 is a hydrogen atom or the like. For the other symbols, refer to the corresponding patent publications.)